1. Field of the Invention
The present invention relates to a structure for protecting scale graduations and, more particularly, to a structure for protecting scale graduations that is suitably applicable to the cases where aged deterioration of the scale used for a linear encoder, arc encoder, rotary encoder and the like is to be avoided, and where output variations caused by the scale are to be reduced.
2. Description of the Related Art
A conventional linear encoder, for example as shown in FIG. 1 illustrating its outline, has a scale 10 and a detector 20 arranged to face each other. The linear encoder detects a relative displacement between the scale 10 and the detector 20 by relatively moving the scale 10 and/or the detector 20 in a measurement direction shown by a double-headed arrow.
In such a conventional scale 10 adapted to the encoder, a pattern of a graduation unit 14 is formed by a deposition technique on a main surface of a scale base 12 made of glass.
In some encoders, in which the scale 10 mainly made of glass and the detector 20 are arranged to face each other, a cover glass 16 is joined with the scale 10 to be a single unit in order to protect the graduation unit 14 of the scale 10 from scratches and stains, as shown in FIG. 2.
However, in order to join the cover glass 16 with the scale 10 to be a single unit as described above, as in FIG. 2 schematically showing the scale 10, an upper adhesive layer 18 is disposed between the top surface portion of the graduation unit 14 and the cover glass 16. In this case, aged deterioration of the graduation unit 14 is caused because a common adhesive agent is mainly made from an acrylic or epoxy resin and is likely to absorb graduations-deteriorating components including moisture and gas from the surrounding atmosphere. In particular, when the graduation unit 14 is made from metal, contrast decreases because its surface tarnishes, and when it is made of a photosensitive material, its properties change, thus resulting in the problem of reducing detection accuracy in both cases.
This problem similarly exists even when a space between the scale base 12 and the cover glass 16 along the entire lateral periphery of the graduation unit 14 is sealed using a periphery adhesive layer 18A including an adhesive agent of the same kind, in addition to the upper adhesive layer 18 between the top surface portion of the graduation unit 14 and the cover glass 16, as shown in FIG. 2.
Moreover, when the cover glass 16 is arranged on and joined with the adhesive layer (agent) 18 formed on the top surface portion of the graduation unit 14, another problem of variations in encoder output, which is outputted from the detector 20 as detection values, is caused due to unevenness in thickness of the adhesive agent existing in measurement (movement) direction orthogonal to the width direction in the drawing.
It should be noted that Japanese Patent Application Laid-Open No. 2002-318137 (hereinafter referred to as Patent Literature 1) discloses a technique of preventing intrusion of harmful coolant, oil and the like upon a scale electrode side by forming a coating layer on the scale electrode (graduation unit) with its pattern formed on a board. However, the similar problem also exists in this scale structure, because an adhesive agent is used to join the coating layer with a scale cover corresponding to the cover glass.